The invention relates to an encoding device comprising a quantizer for converting an input signal into a one-bit encoded signal, which quantizer comprises an input for receiving the input signal and an output for supplying the one-bit encoded signal, and further comprising error-feedback means for feeding back to the quantizer input a quantization-error signal caused by the quantizer, which error-feedback means comprise an at least third-order filter having an input and an output.
Such a device is suitable for use in analog-to-digital and digital-to-analog converters.
In analog-to-digital conversion the bandwidth of the analog signal is usually limited to substantially half the sampling frequency by means of an analog low-pass filter, after which sampling and quantization is effected. In this way an analog audio signal, for example with a frequency of 44.1 kHz, can be sampled and can subsequently be converted into a 16-bit pulse-code-modulated signal.
Such an analog-to-digital conversion imposes very stringent requirements on the analog input filter in view of the required frequency roll-off above the signal band and on the quantizer in view of the accuracy required for 16-bit quantization. In order to enable less stringent requirements to be imposed on the input filter in analog-to-digital converters, it is known to convert the input signal by means of a 1-bit quantizer into a 1-bit signal having a sampling frequency which is substantially higher than approximately twice the highest signal frequency. By means of a decimating filter this 1-bit signal can subsequently be converted into, for example, a 16-bit pulse-code modulated signal having a lower sampling frequency. The decimating filter comprises a digital low-pass filter followed by a down-sampler. The low-pass filter is then required to have a steep roll-off above the signal band, which is easier to realize for a digital filter than for an analog filter. The 1-bit encoding device may then be a device in which the quantization error produced by the quantizer is substracted from the input signal via a loop filter before this signal is applied to the quantizer. If the transfer function of this loop filter is substantially unity within the signal band and rolls off steeply above this band, this ensures that the quantization noise, corresponding to the quantization errors, in the 1-bit encoded signal within the signal band is attenuated at the expense of an increase in quantization noise outside the signal band.
A similar encoding device may also be employed in a digital-to-analog converter in which, for example, a 16-bit pulse-code-modulated signal with a sampling frequency of 44.1 kHz is first converted into a 28-bit encoded signal with a sampling frequency of 176.4 kHz by means of an interpolating filter and is subsequently re-converted into a 1-bit signal by means of the encoding device, the quantization error being substracted from the input signal via a loop filter. Again, this results in a reduction of the quantization noise in the 1-bit signal within the signal band at the expense of an increase of this noise above the signal band. Subsequently, the 1-bit output signal is applied to the actual 1-bit digital-to-analog converter, which can be realized in a comparatively simple manner.
An encoding device of the type defined in the opening paragraph is known from German Patent Specification No. 3,021,021, in which the device is employed in order to reduce the quantization noise in a digital-to-analog converter in a way similar to that described above. Said Patent Specification also states that the reduction of quantization-noise within the signal band increases as the order of the loop filter is higher. The loop filter described in said Patent Specification have a transfer function given by 1-H(Z)=(Z-b).sup.n /Z.sup.n, where n is the order of the loop filter and b is a constant which is substantially equal to unity. Such loop filters provide a satisfactory reduction of quantization noise within the signal band, but this known encoding device has the disadvantage that it exhibits instabilities for loop filters of orders higher than the second order.